The invention is in the field of analog-to-digital converters and more particularly is an analog-to-dogital converter using Josephson devices connected as superconducting interferometers.
Devices which exhibit the Josephson effect are known and such devices have been disclosed as being useful for switching functions. Such devices are cryogenic and have an I-V characteristic which allows them to perform switching functions. The voltage across a device is V=0 for gating currents up to I.sub.m, the critical current. At I.sub.m the voltage jumps to V=(2.DELTA./e).noteq.0 where .DELTA. is the superconducting energy gap. The critical current also varies with magnetic field applied to the Josephson device. The magnetic field is typically applied by means of a control current flowing through a control line overlapping the Josephson device. The critical current, I.sub.m, versus the control current, I.sub.c, is known as the gain curve of the device. A Josephson device can be made to switch from the V=0 to the V=(2.DELTA./e) state by varying the control current I.sub.c. For example, if a control current of I.sub.cl and a gate current of I.sub.gl are applied, and if I.sub.cl causes I.sub.ml to be above I.sub.gl, the device will be in the V=0 state. If the control current is changed to I.sub.c2, which causes I.sub.m2 to be less than I.sub.gl, and I.sub.gl is again applied as the gate current, the device will switch to V=2.DELTA. state. This is shown in FIG. 1b. An Analog-To-Digital Converter Using Josephson Junctions is disclosed by H. H. Zappe to IBM technical disclosure bulletin Vol. 17, no. 10, March 1975. The Zappe device suffers from the difficulty of achieving binary sensitivity ratios due to the use of variable inductance interferometers. Also the Zappe device produces binary output code and leads to large errors in the output due to imperfect alignment of the threshold values.